1. Technical Field of the Invention
The present invention relates to a reflection liquid crystal display such as a display for a mobile terminal device, a terminal display for utilizing various types of media for individuals, a display of a mobile telephone, and a display in an amusement device such as a game machine, a method for producing the same, and a method for driving the same. More specifically, this invention relates to a reflection liquid crystal display having excellent field-of-view angle characteristics, the production process of which is facilitated, a method for producing the same and a method for driving the same.
2. Description of the Related Art
Recently, demand for a reflection liquid crystal display, power consumption of which is reduced, has increased in line with development and diversification of mobile devices. Such a display that enables color display has been increasingly desired for use in, particularly, mobile telephone sets, mobile terminals, and office automation devices. A brighter display is further required in view of mobility, and reasonably wide field-of-view angle characteristics are required. Especially preferable, since narrow field-of-view angle characteristics are desired in an individual use although wide field-of-view angle characteristics are desired where a plurality of people observes a display, a reflection liquid crystal display that enables a changeover between a wide field-of-view angle and a narrow field-of-view angle is desired.
Conventionally, such types are widely used in reflection liquid crystal displays, in which a polarization plate is used as in a supertwisted nematic type (STN type) or twisted nematic type (TN type) that has been widely used in transmission type liquid crystal displays. In the case of reflection type, only a single polarization plate is used, differing from the transmission type. However, since it is necessary to switch reflection light, such a type is common, which is a so called single polarization plate used along with a quarter-wavelength plate as described in, for example, T. Sonehara et al., Japan Display 1989, Page 192 (1989).
A description is given of a display principle of a single polarization plate type, taking a normally white type, which has been most widely used, as an example (Prior art example 1). FIG. 1A and FIG. 1B are views showing the display principle of a reflection type liquid crystal element of a prior art single polarization plate type, wherein FIG. 1A shows an example in the case of white display, and FIG. 1B shows an example in the case of black display. Also, FIG. 1A and FIG. 1B show only optical elements of the reflection liquid crystal display.
As shown in FIG. 1A, incident non-polarized light 1107 is brought into collision with a reflection plate 1104 after having passed through a quarter-wavelength plate 1102 of a wide-band and passed through a liquid crystal layer 1103 and returns in its inverted optical route, and, when the light passes through the polarization plate 1101, it enters eyes of people, wherein the light can be recognized as an image. At this time, by changing the state of polarization by utilizing electro-optical effects of liquid crystal, switching of the reflection light can be carried out. First, the incident non-polarized light 1107 enters the polarization plate 1101 and is converted to light polarization having a specified vibration direction. At this time, the polarization plate is set so that the outgoing light becomes P-polarized light 1105. And, if the optical axis of the quarter-wavelength plate 1102 is disposed so that it forms 45 degrees with respect to the transmission axis of the polarization plate, the light that has passed through the quarter-wavelength plate 1102 becomes rightward circular-polarized light 1106 and enters the liquid crystal layer 1103. In either the TN mode or the STN mode, retardation of the liquid crystal 1103 is set so that it gives λ/4, that is, a phase difference π/2 when no voltage is applied. Therefore, light that has passed through the liquid crystal layer 1103 again becomes P-polarized light 1105 and reaches the reflection plate 1104. In the reflection, since the P-polarized light 1105 is reflected as it is P-polarized light 1105, it returns in a completely inverted optical path of the path along which it entered, and is converted to rightward circular-polarized light 1106 by the liquid crystal layer 1103. Further, the light becomes P-polarized light 1105 by the quarter-wavelength plate 1102 and is caused to radiate from the polarization plate 1101 as it is P-polarized light 1105. That is, white display is enabled in a state where no voltage is applied onto the liquid crystal.
As shown in FIG. 1B, if voltage is applied to the liquid crystal layer 1103 and liquid crystal is erected so that the liquid crystal molecules become perpendicular with respect to its substrate, the retardation of the liquid crystal layer 1103 becomes almost zero, and a phase difference 0 is given. That is, the liquid crystal layer 1103 does not give any influence to the state of polarization. In this state, where incident non-polarized light 1107 enters the polarization plate 1101, light that has passed through the polarization plate 1101 and quarter-wavelength plate 1102 becomes rightward circular-polarized light 1106 as described above. Herein, since voltage is applied onto the liquid crystal layer 1103, the liquid crystal layer 1103 does not change the state of polarization, and the rightward circular-polarized light 1106 passes through the liquid crystal layer 1103 as it is the rightward circular-polarized light 1106 and enters the reflection plate 1104. Since the advancement direction of the light is inverted by reflection, the rightward circular-polarized light 1106 becomes a leftward circular-polarized light 1108 and returns. Since the liquid crystal layer 1103 also does not change the state of polarization, light that passed through the liquid crystal layer 1103 enters the quarter-wavelength plate 1102 as it is a leftward circular-polarized light, and it becomes S polarization 1109 whose direction of polarization is different by 90 degrees from the P-polarized light 1105, wherein the light enters the polarization plate 1101. Since the transmission axis of the polarization plate 1101 is set so that it can make the P-polarized light pass therethrough, wherein the S polarization 1109 cannot pass through the polarization plate 1101, and it is displayed as black. Depending upon the intensity of application voltage, the retardation of the liquid crystal layer 1103 can be varied, wherein intermediate colors can be displayed.
Also, Japanese Unexamined Patent Application No. Hei-10-20323 (hereinafter called a “prior art example 2”) has disclosed a liquid crystal display whose production is facilitated and field-of-view angle characteristics are excellent. In the prior art example 2, a liquid crystal layer in which two or more types of slight areas coexist is placed between two substrates, and has an electrode having an opening formed on at least one substrate, and a second electrode (control electrode) secured in the opening, wherein a voltage that is higher than the voltage applied between the electrode having the opening and the electrode opposed thereto is applied between the control electrode and the electrode opposed thereto, thereby securing a wide field-of-view angle.
Further, Japanese Unexamined Patent Publication No. Hei-7-239471 (hereinafter called a “prior art example 3”) discloses the use of a cholesteric material layer and phase plate, that act as a reflection layer, for the purpose of improving the brightness and color purity of a reflection liquid crystal display. In the prior art example 3, the upper and lower substrates are disposed so as to be opposed to each other, a liquid crystal layer is placed and secured between these two substrates, and the prior art example 3 comprises a phase plate disposed on the opposite side of the upper substrate liquid crystal layer, an upper polarization plate disposed further thereon, a cholesteric material layer that is disposed on the surface of the lower substrate opposed to the upper substrate and is disposed between the surface and the liquid crystal layer, and an optical absorption layer formed at the opposite side of the lower substrate liquid crystal layer. Thus, the cholesteric material layer is formed in liquid crystal cells and is used as a color filter, whereby shadows in the dark display portion can be removed.
In addition, a liquid crystal display in which a wide field-of-view angle and a narrow field-of-view angle are changed over is disclosed in Japanese Unexamined Patent Application Nos. Hei-6-59287 and Hei-10-197844 (hereinafter respectively called a “prior art example 4” and a “prior art example 5”).
In the prior art example 4, the field-of-view angle of transmission liquid crystal cells is changed over by adjusting the outgoing light, utilizing a guest-host liquid crystal or grating. Also, in the prior art example 5, such a method is disclosed, in which the reflection type and transmission type are changed over by utilizing transmission and dispersion of macromolecular dispersion liquid crystal, and the changeover of the field-of-view angle of a liquid crystal display is carried out by utilizing the guest-host liquid crystal.
However, in the case of displaying by means of the mode of prior art example 1, as has been made clear in FIG. 1A, since the light that enters the liquid crystal layer 1103 in bright display is based on linear polarization, in the TN mode or STN mode, it is necessary to set the rubbing direction and polarization direction, so that they are made coincident with each other or different by 90 degrees from each other, in order to obtain a high transmittivity, and it is necessary to accurately control the rubbing direction and arrangement of the polarization plate 1101 and a wide-band quarter-wavelength plate 1102. In addition, if a perpendicular orientation mode and amorphous TN mode, which can shorten the production process without requiring any rubbing, are used, a dark display portion is securely produced in a bright display state, wherein sufficient brightness cannot be obtained. Further, another problem occurs in that, since the field-of-view angle is unitarily determined by design of a reflection plate, the wide field-of-view angle and narrow field-of-view angle cannot be changed over.
Still further, in the art described in the prior art example 2, such problem occurs in that a voltage must be applied to the second electrode in order to control its drive and a wide field-of-view angle and narrow field-of-view angle cannot be switched.
In addition, in the art described in the prior art example 3, in the phase plate, no consideration is taken with respect to the relationship between the direction of liquid crystal orientation and the direction of polarization of reflection light that enters the liquid crystal layer in the TN and STN modes, and no device is provided so that brightness can be secured in regard to fluctuations of a process such as a rubbing direction, etc. In particular, even if a perpendicular orientation mode or amorphous TN mode, which can shorten the production process not requiring any rubbing, is used, no consideration is provided with respect to a method and/or construction of securing sufficient brightness. Therefore, sufficient brightness cannot be secured by such a simple process. Also, where the cholesteric material layer is used as a reflection layer, since the range of field-of-view angle that is capable of observing selection reflection is narrow, it is necessary to further widen the field-of-view angle in practice. However, a problem occurs in that a wide field-of-view angle and a narrow field-of-view angle cannot be changed over.
Still further, in the prior art example 4, it is disclosed only that the outgoing light is adjusted by utilizing a guest-host liquid crystal or grating in order to adjust the field-of-view angle of transmission liquid crystal cells. No disclosure is provided with respect to a reflection type. Furthermore, where the field-of-view angle is changed over by the system disclosed in the prior art example 4, a problem occurs in that, although it is possible to only narrow and/or limit, in a certain area, the field-of-view angle of liquid crystal cells to be used, it is not possible to widen the field-of-view angle of the liquid crystal cells.
In addition, in the prior art example 5, as in the prior art example 4, although it is possible to only narrow and/or limit, in a certain area, the field-of-view angle of liquid crystal cells to be used, it is not possible to widen the field-of-view angle of the liquid crystal cells. Therefore, it is necessary to use a mode in which the field-of-view angle of the liquid crystal cells is wide. A liquid crystal mode having practical brightness and high contrast in the reflection type is only a single polarization plate type of TN as in the prior art example 1. However, another problem occurs in that the field-of-view angle in the mode is narrow, and a practically sufficient field-of-view angle cannot be obtained by the system by which the field-of-view angle is further narrowed.
Therefore, as described above, it is difficult to secure sufficient brightness by a single polarization plate type in which a conventional TN or STN mode is employed. Still further, it is necessary to accurately control the rubbing direction, etc., and tolerance is narrow with respect to fluctuations in the process. In particular, it is impossible to secure sufficient brightness with respect to the perpendicular orientation not requiring any rubbing and the mode of amorphous TN, etc. In addition, still another problem occurs in that the necessary field-of-view angle in practice cannot be secured, and the field-of-view angle cannot be changed over.